chemical reactors, chemical engineering

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Chemical EngineeringTypes of Chemical Reactors

IntroductionReactor is the heart of Chemical Process.A vessel designed to contain chemical reactions is called a reactor.An industrial reactor is a complex chemical device in which heat transfer, mass transfer, diffusion and friction may occur along with chemical with the provisions of safety and controls

Basic PrincipleAll chemical processes are centered in a chemical reactor. The design of a chemical reactor Is the most important factor in determining the overall process economics.

Basics for DesignReaction TypeRemoval/addition of heatNeed for catalystPhases involveThe mode of temperature and pressure control.Production capacity or flow Residence timeContact/mixing between the reactants

Reaction TypesDirect Combination or Synthesis ReactionA + B = AB Chemical Decomposition or Analysis ReactionAB = A + B

Reaction TypesSingle Displacement or Substitution ReactionA + BC = AC + B Metathesis or Double Displacement ReactionAB + CD = CB

In addition to the basic data, include:A heat and mass transfer characteristicsPhysical, chemical and thermodynamic properties of components taking part in the reaction.Corrosion- erosion characteristics of any potential hazard associated with reaction system. Reaction Rate

Endothermic/Exothermic Reactions within- heating describes a process or reaction that absorbs energy in the form of heat.Release energy in the form of heat, light, or sound. S > 0 H < 0

Reaction Rate Speed at which a chemical reaction proceeds, in terms of amount of product formed or amount of reactant consumed per unit time.

Factors Influencing Reaction Rate Concentration The nature of reaction Temperature Pressure Catalyst

Modeling Principle:

Inputs + Sources = Output + Sink + Accumulations

Basic Reactor ElementMaterial BalancesHeat Transfer and Mass Transfer

Material BalancesAlso called mass balance.Is an application of conservation of mass to the analysis of physical systems. The mass that enters a system must, by conservation of mass, either leave the system or accumulate within the system .

Mass Balance

Mathematically the mass balance for a system without a chemical reaction is as follows Input = Output + Accumulation

Mass Transfer

Is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical system.Transfer of mass from high concentration to low concentration.

Heat TransferIs the transition of thermal energy from a heated item to a cooler item.Transfer of Thermal Energy

Modes Of Heat Transfer jacket, internal coils, external heat exchanger, cooling by vapor phase condensationfired heater.

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Reactor TypesThey can be classified according to the;Mode of operationEnd use applicationNo of PhasesA catalyst is used

Classification by Mode of OperationBatch ReactorsContinuous reactorsSemi-batch reactors

Batch ReactorA batch of reactants is introduced into the reactor operated at the desired conditions until the target conversion is reached.Batch reactors are typically tanks in which stirring of the reactants is achieved using internal impellers, gas bubbles, or a pump-around loop where a fraction of the reactants is removed and externally recirculated back to the reactor.

Batch ReactorsTemperature is regulated via internal cooling surfaces (such as coils or tubes), jackets, reflux condensers, or pump-around loop that passes through an exchanger. Batch processes are suited to small production rates, too long reaction times, to achieve desired selectivity, and for flexibility in campaigning different products

Batch Reactor

Applications of Batch reactorFermentation of beverage productsWaste water treatment

Continuous ReactorsReactants are added and products removed continuously at a constant mass flow rate. Large daily production rates are mostly conducted in continuous equipment.

Continuous ReactorsCSTRPlug Flow ReactorTubular flow reactor

CSTRA continuous stirred tank reactor (CSTR) is a vessel to which reactants are added and products removed while the contents within the vessel are vigorously stirred using internal agitation or by internally (or externally) recycling the contents. CSTRs may be employed in series or in parallel.

CSTRResidence time average amount of time a discrete quantity of reagents spend inside the tankResidence time = volumetric flow rate volume of the tank At steady state, the flow rate in must be equal the mass flow rate out.

CSTR ApplicationsContinuous stirred-tank reactors are most commonly used in industrial processing, primarily in homogeneous liquid-phase flow reactions, where constant agitation is required. They may be used by themselves, in series, or in a battery.Fermentors are CSTRs used in biological processes in many industries, such as brewing, antibiotics, and waste treatment. In fermentors, large molecules are broken down into smaller molecules, with alcohol produced as a by-product.

Advantages/Disadvantages of CSTRGood temperature control is easily maintainedCheap to constructReactor has large heat capacityInterior of reactor is easily accessedDisadvantage:Conversion of reactant to product per volume of reactor is small compared to other flow reactors

Plug Flow Reactor Plug flow, or tubular, reactors consist of a hollow pipe or tube through which reactants flow. Pictured below is a plug flow reactor in the form of a tube wrapped around an acrylic mold which is encased in a tank. Water at a controlled temperature is circulated through the tank to maintain constant reactant temperature.

Plug Flow ReactorReagents may be introduced into the reactors inletAll calculations performed with PFRs assume no upstream or downstream mixing. Has a higher efficiency than a CSTR at the same value

Schematic Diagram of Plug Flow Reactor

Applications of Plug flow reactorPlug flow reactors have a wide variety of applications in either gas or liquid phase systems. Common industrial uses of tubular reactors are in gasoline production, oil cracking, synthesis of ammonia from its elements, and the oxidation of sulfur dioxide to sulfur trioxide.

Tubular Flow ReactorA tubular flow reactor (TFR) is a tube (or pipe) through which reactants flow and are converted to product. The TFR may have a varying diameter along the flow path. In such a reactor, there is a continuous gradient (in contrast to the stepped gradient characteristic of a CSTR-inseries battery) of concentration in the direction of flow. Several tubular reactors in series or in parallel may also be used. Both horizontal and vertical orientations are common

Tubular Flow ReactorChemical reactions take place in a stream of gas that carries reactants from the inlet to the outlet

The catalysts are in tubes Uniform loading is ensured by using special equipment that charges the same amount of catalyst to each tube at a definite rate.

Semi Batch ReactorSome of the reactants are loaded into the reactor, and the rest of the reactants are fed gradually. Alternatively, one reactant is loaded into the reactor, and the other reactant is fed continuously. Once the reactor is full, it may be operated in a batch mode to complete the reaction. Semi-batch reactors are especially favored when there are large heat effects and heat-transfer capability is limited. Exothermic reactions may be slowed down and endothermic reactions controlled by limiting reactant concentration.

Semi Batch reactorsIn bioreactors, the reactant concentration may be limited to minimize toxicity.Other situations that may call for semibatch reactors include control of undesirable by-products or when one of the reactants is a gas of limited solubility that is fed continuously at the dissolution rate.

Classification By End UseChemical reactors are typically used for the synthesis of chemical intermediates for a variety of specialty (e.g., agricultural, pharmaceutical) or commodity (e.g., raw materials for polymers) applications.

Classification by End usePolymerization ReactorsBio-reactorsElectrochemical Reactors

Polymerization ReactorsPolymerization reactors convert raw materials to polymers having a specific molecular weight and functionality. The difference between polymerization and chemical reactors is artificially based on the size of the molecule produced.

Bio ReactorsBioreactors utilize (often genetically manipulated) organisms to catalyze biotransformations either aerobically (in the presence of air) or an-aerobically (without air present).

Electrochemical reactorsElectrochemical reactors use electricity to drive desired reactions. Examples include synthesis of Na metal from NaCl and Al from bauxite ore. A variety of reactor types are employed for specialty materials synthesis applications (e.g., electronic, defense, and other).

Classification by PhaseDespite the generic classification by operating mode, reactors are designed to accommodate the reactant phases and provide optimal conditions for reaction.Reactants may be fluid(s) or solid(s), and as such, several reactor types have been developed. Single phase reactors are typically gas- (or plasma- ) or liquid-phase reactors.Two-phase reactors may be gas-liquid, liquid-liquid, gas-solid, or liquid-solid reactors.

Classification by phaseMultiphase reactors typically have more than two phases present. The most common type of multiphase reactor is a gas-liquid-solid reactor; however, liquid-liquid-solid reactors are also used. The classification by phases will be used to develop the contents of this se

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